Antenna devices are known. These include wire antennae and waveguide horns. An antenna is driven by a power source via an impedance matching network. The network is required because typical solid state power sources such as Gunn or impatt diodes have impedances much lower than that of a wire antenna or waveguide horn. The matching network is not incorporated monolithically in the solid state power source structure since this is not necessarily technically feasible and is wasteful of valuable semiconductor material in any event Antenna devices are accordingly usually of hybrid form. However, the reactance of the power source is then a function of bond wire connections and the like. The result is that solid state power sources require individual manual adjustment. At higher frequencies in particular, matching requires the use of waveguide cavities which are heavy and bulky relative to the power source or antenna. Moreover, the required degree of mismatch reduction reduces power amplifier bandwidth.
To avoid the need for an impedance matching network, microstrip patch antennae have been developed. Such an antenna typically consists of a planar rectangular patch of metal on one surface of a dielectric substrate sheet, the other surface bearing a ground plane. The antenna impedance can be arranged to allow a power source to be integrated directly into the antenna structure without an intervening matching network. However, it is found that radiative efficiency is low and bandwidth severely limited for such an antenna as compared to conventional types. Radiative efficiency is low because much of the energy radiated by a patch antenna of known kind is trapped within the substrate layer, and only a small proportion is radiated into free space. Similar effects have been analysed by Brewitt-Taylor, Gunton and Rees in Electronics Letters, 1st Oct. 1981, Vol 17, pp 729-731.
It is an object of the invention to provide an alternative form of antenna device.